If you still have problems: Check if your machine isn't behind some and try to set the necessary settings in File->Preferences->Proxy. Alternatively, bypass the proxy if you can do so.

Reason for step 4:
After doing steps 1 to 3 I could ping servers. However, I was still having problems using
yum (I can ping mirrorlist.centos.org, but yum still gives that error). This was due to my local machine being on a network that used a proxy with authentication using my windows logon. I fixed this by going off the local network onto the guest wifi which doesn't happen to go over that proxy.